1. Field of the Invention
The present invention relates to a photodetector amplifier circuit.
2. Description of Related Art
FIG. 3 illustrates a circuit diagram of a photodetector amplifier circuit 1 disclosed in Japanese Unexamined Patent Application Publication No. 11-41036 as a known art. As shown in FIG. 3, the photodetector amplifier circuit 1 includes a photodiode PD1, amplifiers 2 and 3, and feedback resistors R1 to R4. The feedback resistor R1 is connected between an input and an output of the amplifier 2. The photodiode PD1 is connected between a power supply voltage terminal Vcc and the input of the amplifier 2. The feedback resistor R2 is connected between the output of amplifier 2 and an input of the amplifier 3. The feedback resistors R3 and R4, which are connected in series, are connected between the input and an output of the amplifier 3. An output terminal Vout1 is connected to the output of the amplifier 3.
The photodetector amplifier circuit 1 converts a photocurrent Ipd, which is an output current from the photodiode PD1, into a voltage by the feedback resistor R1 and the amplifier 2. Further, the photodetector amplifier circuit 1 amplifies the output voltage by a gain of (R3+R4)/R2 by the amplifier 3 and the feedback resistors R2 to R4. Then the photodetector amplifier circuit 1 outputs the amplified voltage to the output terminal Vout. For convenience, the above codes “VCC” and “Vout1” indicate the names of terminals as well as a power supply voltage and an output voltage, respectively. Further, the codes “R1” to “R4” indicate the names of resistance devices as well as their resistance values. Hereafter, codes of the terminals and resistance devices mentioned in this document also indicate voltages of the terminals and resistance values.
The abovementioned photodetector amplifier circuit 1 is mainly for photodetector ICs which are used as a pick-up in the field of storage such as CDs and DVDs. If the photodetector amplifier circuit 1 is used as a photocoupler, a high driving capability of the device in the subsequent stage device is required. Thus a base of an open collector output is connected to the output terminal Vout. This improves the driving capability.
An example of such circuit configuration as a photocoupler 10 is shown in FIG. 4. As shown in FIG. 4, the photocoupler 10 includes a light-emitting diode LED1, a photodetector amplifier circuit 1, a resistor R6, and an NPN transistor Q1. The photodetector amplifier circuit 1 has the same configuration as the one described above. It is noted that the resistors R3 and R4 are represented as a resistor R5. The output terminal Vout1 is referred to as a node F. The light-emitting diode LED1 is connected between signal input terminals Sin1 and Sin2. One end of the resistor R6 is connected to the node F. As for the NPN transistor Q1, a base is connected to the other end of the resistor R6, a collector is connected to an output terminal Vout2, and an emitter is connected to a ground voltage terminal GND. The NPN transistor Q1 has an open collector configuration. Thus, a pull-up resistor R7 is connected between the power supply voltage Vcc and the output terminal Vout2. The circuit of the above configuration is packaged as the photocoupler 10.
The operation of the photocoupler 10 is described hereinbelow. First, an electric signal is input to a primary light-emitting diode LED1 so that the light-emitting diode LED1 emits light. The photodiode PD1 of the second photodetector amplifier circuit 1 receives the emitted light to generate the photocurrent Ipd. The operation of the photodetector amplifier circuit 1 is as described above. The NPN transistor Q1 is driven according to a potential of the node F, which is an output of the photodetector amplifier circuit 1.
In response to an input of a signal to the light-emitting diode LED1, the potential level of the node F, which is the output of photodetector amplifier circuit 1, becomes high. This makes the NPN transistor Q1 turn on, which is connected to the node F via the resistor R6, turn on. Then the potential level of the output terminal Vout2 becomes low.
Conversely, if there is no signal input to the light-emitting diode LED1, the potential level of a node A, which is an output of the photodetector amplifier circuit 1, becomes low. This makes the NPN transistor Q1 turn off. The NPN transistor Q1 is connected to the node F via the resistor R6. Then the potential level of the output terminal Vout2 becomes high. By repeating such operation, signals input to the primary side are transmitted to the secondary side of the output terminal Vout2. Since the primary and the secondary side transmit signals via light, they are electrically insulated within the photocoupler 10.
FIG. 5 shows the circuit configuration of the common amplifier 3. As shown in FIG. 5, the amplifier 3 includes NPN transistors Q2 and Q3, and resistors R8 and R9. The resistor R8 and the NPN transistor Q2 are connected in series between the power supply voltage terminal Vcc and the ground voltage terminal GND. A base of the NPN transistor Q2 corresponds to the input of the amplifier 3. The NPN transistor Q3 and the resistor R9 are connected in series between the power supply voltage terminal Vcc and the ground voltage terminal GND. A base of the NPN transistor Q3 is connected to an intermediate node between the resistor R8 and the NPN transistor Q2. An intermediate node between the NPN transistor Q3 and the resistor R9 corresponds to the output of the amplifier 3. Accordingly, the NPN transistor Q3 has an emitter follower configuration. The operation of the amplifier is well known, thus the explanation is omitted here.